This invention relates to cathode ray tubes, and particularly to shadow mask type color picture tubes comprising a plurality of electron guns, a multi-apertured shadow mask, and a mosaic screen of systematically arranged color phosphor areas, such as dots or lines.
The standard shadow mask color picture tube, as presently manufactured and used in most color TV receivers, comprises a glass envelope including a large curved faceplate section or panel containing a curved mosaic phosphor screen and a curved shadow mask, and a funnel section including a small neck section containing three laterally spaced electron guns positioned at the three corners of an equilateral triangle. The tube is provided with internal and external means for converging the three beams from the three guns at or near the screen and external means for scanning the three beams in a rectangular raster over the mask and screen. The screen is made up of a multiplicity of small circular deposits or dots of red, blue and green emitting phosphor material arranged in a hexagonal pattern in triangular groups or triads of red, blue and green dots in each triad. The shadow mask is a thin metal member having a multiplicity of circular apertures, one for each dot triad, also arranged in a hexagonal pattern. The mask is removably mounted, as by means of studs and leaf springs, on the panel next to the screen. The spacing between the mask and screen, which averages about one-half inch, is chosen at each radial distance to provide desirable grouping of the electron beam spots on the screen at each radial distance from the center during operation of the tube.
Each color pattern of the mosaic screen is deposited on the glass faceplate by a direct photographic process wherein a photosensitive coating is exposed through the mask apertures by actinic rays from a light source located at, or related to, the source of one of the electron beams, and the coating is then developed, by washing off the unhardened unexposed portions, leaving the desired pattern of exposed hardened dots. This process is repeated for each color pattern. The color phosphor powder may be mixed directly with each photosensitive coating before application to the faceplate, or applied to the pattern after exposure.
During the exposure of the coating, the exposed dot portions are caused to grow so that the final dots are several mils larger than the mask apertures. In tube operation, the electron beam portions which pass through the apertures are also caused to grow but to a lesser extent, so that the penumbra of each beam spot on the screen is also larger than the mask aperture but smaller than the phosphor dot that it impinges. The difference in diameter between the beam spot and its dot, minus the total overlap (if any) of the dot with the adjacent dots on opposite sides thereof, is known as the "white uniformity tolerance", or "leaving tolerance". It is the total range of movement of a particular beam spot relative to its color dot without reducing the amount of light of that color emitted. In a standard shadow mask tube, where the spot is smaller than the dot, this tolerance is called a "positive" tolerance. Another tolerance to be considered is the "purity tolerance" or "clipping tolerance". This is the total range of movement of a particular spot relative to its own color dot without impinging on (clipping) an adjacent dot of a different color, and is equal to twice the distance between the centers of two adjacent dots minus the diameter of the dot minus the diameter of the beam spot (penumbra), for dots having equal size and spacing. As an example, in the central region of a 25 inch 90.degree. rectangular shadow mask color tube, RCA-25AJP22, manufactured by applicant's assignee, the mask apertures have a diameter of 12 mils and a spacing of 28.1 mils; the phoshor dots have a maximum diameter of 17.8 mils and an average center-to-center spacing of 16.95 mils (overlapping dots); in which case the beam spot diameter is about 13.7 mils; the minimum leaving tolerance is about 2.4 mils, and the clipping tolerance is about 2.4 mils. Since the mask apertures are only 12 mils, the beam transmission at the center of the standard mask is only about 16.5%, which limits the available light output or brightness of the screen. Moreover, the light output of the tube is further reduced by the necessity for using a gray glass faceplate having a light transmission of only 41% in order to maintain acceptable contrast by minimizing internal halation effects and reflections of ambient light from the screen. The contrast is the ratio of the highlight (brightest portion) of the screen to the lowlight (unexcited portion).
As the beams are deflected (scanned) toward the edge of the screen they are subjected to electrical and magnetic effects, such as axial shift of the centers of deflection with increasing deflection angle, outward shift of the deflection centers with dynamic convergence, astigmatic effects in the deflection yoke, and the earth's magnetic field, which do not affect the light rays during screen printing, and hence, which tend to cause misregister of the beam spots with their corresponding dots during tube operation. Although most of these effects are compensated for by using a special light refracting member or lens, e.g., as disclosed in Epstein et al. U.S. Pat. No. 2,885,935, dated May 13, 1959, or Morrell et al. 3,282,691, dated Nov. 1, 1966, in the screen printing operations, some misregister errors remain. Therefore, greater tolerances are required at the edges and corners than at the center of the screen. For this reason, the mask apertures are graded in size from a maximum at the center to a minimum at the corners, and substantially more growth of the dots is produced at the edge than at the center during screen printing. In the standard 25 inch tube referred to above, the apertures in the corners of the rectangular mask at a distance of 11.25 (inches) from the center have a diameter of 10 mils and a spacing of 27.7 mils, the phosphor dots have a diameter of 16 mils and an average center-to-center spacing of 16 mils, and the beam spot diameter is about 11.6 mils; and hence, the leaving and clipping tolerances are both about 4.4 mils. The beam transmission of the mask at the corners is only about 12%, so that the light output is about 30% less than at the center of the screen.
If a more transparent faceplate is used, to increase the light output or brightness, the result is an undesirable net decrease in contrast, because the reflectivity of the screen varies as the square of the glass transmission. On the other hand, if the mask apertures are made larger, to increase the light output and the contrast, the leaving and clipping tolerances are both decreased, which has heretofore been considered intolerable.
U.S. Pat. No. 2,842,697 to F. J. Bingley describes a color picture tube of the line screen type, e.g., a sensing tube with indexing strips, in which the different color emitting phosphor strips are spaced from each other to increase the color purity tolerance, and also permit the use of a larger beam spot without having the spot overlap more than one (desired) color strip at a time, to increase the brightness of the reproduced image. Moreover, the spaces between the color strips are filled with opaque and non-reflecting material to reduce halations and the reflectivity of the screen to ambient light, and thereby increase the contrast of the image. Since the beam spot is wider than each color phosphor strip, Bingley's line screen tube is a negative tolerance tube. In columns 8 to 10, Bingley described (without illustration) his invention as applied to "the conventional so-called `aperture mask` type of color television display tube". He suggested depositing opaque and non-reflective material on the faceplate in a pattern (matrix) corresponding to the interstices between the phosphor dots of the completed screen, and depositing the respective sets of phosphor dots in the spaces in the deposited pattern of opaque material. He stated that the phosphor dots may be slightly larger than the holes in the opaque pattern provided that they do not encroach on the adjacent holes of different color dots. Thus, the holes must be spaced apart. The patent is silent as to whether the mask apertures are larger or smaller than the holes in the opaque pattern. However, it may be assumed that Bingley's shadow mask embodiment is a negative tolerance tube like the line screen embodiment, and hence, that the mask apertures must be large enough to produce beam spots larger than the holes in the opaque pattern. Bingley's shadow mask embodiment is described more in detail in U.S. Pat. No. 3,146,368 to J. P. Fiore and S. H. Kaplan. In this patent, the mask apertures as well as the beam spots are described as being larger than the color phosphor dots as shown in FIGS. 2 and 4. Because of this negative tolerance relationship, it is difficult to deposit the dots and surrounding non-reflective matrix material on the faceplate by direct photographic methods without temporarily changing the effective size of the mask apertures during the screen printing operation, because of the normal tendency of the dots to grow in size during light exposure.